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Show 56 SPECIES OF LARGE GENERA [CHAP. II. To test the truth of this anticipation I have arranged the plants of twelve countries, and the coleopterous insects of two districts, into two nearly equal masses, the species of the larger genera on one side, and those of the smaller genera on the other side, and it has invariably proved to be the case that a large proportion of the species on the side of the larger genera present varieties, than on the side of the smaller genera. Moreover, the species of the large genera whicli present any varieties, invariably present a larger average number 0f varieties than do the species of the small genera. Both these results follow when another division is made, and when all the smallest genera, with from only one to four species, are absolutely excluded from the tables. These facts arc of plain signification on the view that species are only strongly marked and permanent varieties ; for wherever 1nany species of the same genus have been formed, or where, if we may use the expression, the manufactory of species has been active, we ought generally to find the 1nanufactory still in action, more especially as we have every reason to believe the process of manufacturing new species to be a slow one. And this certainly is the case, if varieties be looked at as incipient species; for my tables clearly show as a general rule that, wherever many species of a genus have been formed, ·the species of that genus present a number of varieties, that is, of incipient species, beyond the average. It is not that all large genera are now varying much, and are thus increasing in the number of their species, or that no s1nall genera are now varying and increasing ; for if this had been so, it would have been fatal to my theory; inasmuch as geology plainly tells us that small genera have in the lapse of time often increased greatly in size; and that large genera have often come to their maxima, declined, . and disappeared. All that we want to show is, that where 1nany species of a genus have been formed, on an average many are still forming ; and this holds good. There are other relations between the species of large genera and their recorded varieties which deserve notice. We have seen that there is no infallible criterion by which CHAP. II.)' RESEMBLE VARIETIES. 57 to distinguish species and well-marked varieties; and in those cases in which intermediate links have not been found between doubtful forms, naturalists are compelled to con1e to a determination by the amount of difference between thmn, judging by analogy whether or not the amo.unt suffices to raise one or both to the rank of species. I-I~nc? t~e amo.unt of difference is one very important crrterion 1n settling whether two forms should be ranked as species or varieties. Now Fries has remarked in regard to plants, and w ·estwood in regard to insects that in large genera the amount of difference between th~ species is often exceedingly small. I have endeavoured to test this nuinerically by averages, and, as far as my imperfect results go, they always confirm the view. I have also consulted so1ne sagacious and 1nost experienced observers, and, after deliberation, they concur in this view. In this respe~t, .therefore, the species of t!1e larger genera resemble varieties, more than do the specws of the smaller genera. <?r the c~se may be put in another way, and it may be said, that 1n the larger genera, in ·which a number of varieties or incipient species greater than the average are now manufacturing, many of the species already 1nanufactured still to a certain extent resemble varieties for they differ from each other by a less than usuai an1ount of difference~ Moreover, the species of the larger genera are related to each other, in the same manner as the varieties of any one species are related to each other. No naturalist pretends that all the species of a genus are equally distinct from each other; they may generally be divided into subgenera, or sections, or lesser groups. As Fries has well remarked, little groups of species are generally clustered like satellites around certain other species. And what are varieties but groups of forn1s, unequally related to each other, and clustered round certain forms ?-that is, round their parent-species? Undoubtedly there is one most important point of difference between varieties and species ; namely, that the amount of difference between varieties, when compared with each other or with their parentspecies, is much less than that between the species of the 3* |
